The reforming of petroleum hydrocarbon streams is an important petroleum refining process which is employed to provide high octane hydrocarbon blending components for gasoline. The process is usually practiced on a straight run naphtha fraction which has been hydro-desulfurized. Straight run naphtha is typically highly paraffinic in nature, but may contain significant amounts of naphthenes and minor amounts of aromatics or olefins. In a typical reforming process, the reactions include dehydrogenation, isomerization, and hydrocracking. The dehydrogenation reactions typically will be the dehydroisomerization of alkylcyclopentanes, to aromatics, the dehydrogenation of paraffins to olefins, the dehydrogenation of cyclohexanes to aromatics, and the dehydrocyclization of paraffins to aromatics. The aromatization of the n-paraffins to aromatics is generally considered to be the most important because of the high octane of the resulting aromatic product compared to the low octane ratings for n-paraffins. The isomerization reactions include isomerization of n-paraffins to isoparaffins, and the isomerization of substituted aromatics. The hydro-cracking reactions include the hydrocracking of paraffins and hydrodesulfurization of any sulfur which is remaining in the feedstock.
It is well known in the art that several catalysts are capable of reforming petroleum naphthas and hydrocarbons that boil in the gasoline boiling range. Examples of known catalysts useful for reforming include platinum and optionally rhenium or iridium on an alumina support, platinum on type X and Y zeolites, platinum on intermediate pore size zeolites as described in U.S. Pat. No. 4,347,394 and platinum on cation exchanged type L zeolites. U.S. Pat. No. 4,104,320 discloses the dehydrocyclization of aliphatic hydrocarbon to aromatics by contact with a catalyst comprising a Type L zeolite containing alkali metal ions and a Group VIII metal such as platinum.
The conventional reforming catalyst is a bifunctional catalyst which contains a metal hydrogenation-dehydrogenation component which is usually dispersed on the surface of a porous inorganic oxide support, usually alumina. Platinum has been widely used commercially in the production of reforming catalysts, and platinum on alumina catalysts have been commercially employed in refineries for the past few decades. More recently, additional metallic components have been added to the platinum to further promote the activity or selectivity, or both. Examples of such metallic components are iridium rhenium, tin and the like. Some catalysts possess superior activity, or selectivity, or both as contrasted with other catalysts. Platinum-rhenium catalysts, for example, possess high selectivity in comparison to platinum catalysts. Selectivity is generally defined as the ability of the catalyst to produce yields of C.sub.5+ liquid products with concurrent low production of normally gaseous hydrocarbons.
There exist several processes for dividing naphtha feedstreams into a higher boiling and a lower boiling cut and reforming these cuts separately. U.S. Pat. No. 2,867,576 discloses separating straight run naphtha into lower and higher boiling cuts, in which the higher boiling cuts are reformed with a hydrogenation-dehydrogenation catalyst with the liquid reformate produced being routed to an aromatics separation process. The paraffinic fraction obtained from the separation process is blended with the lower boiling naphtha fraction and the resulting blend is reformed with a reforming catalyst which may or may not be the same type employed in reforming the high boiling cut.
U.S. Pat. No. 2,944,959 discloses fractionating a full straight run gasoline into a light paraffinic fraction, C.sub.5 and C.sub.6, which is hydroisomerized with hydrogen and a pt-alumina catalyst, a middle fraction which is catalytically reformed with hydrogen and a pt-alumina catalyst, and a heavy fraction which is catalytically reformed with a molybdenum oxide catalyst and recovering the liquid products. U.S. Pat. Nos. 3,003,949, 3,018,244 and 3,776,949 also disclose fractionating a feed into a C.sub.5 and C.sub.6 fraction which is isomerized and a heavier fraction which is reformed.
Other processes for dividing feedstocks and separately treating them include: U.S. Pat. Nos. 3,172,841 and 3,409,540 which disclose separating fraction of a hydrocarbon feedstock and catalytically reforming various fractions of the feed; U.S. Pat. No. 4,167,472 which discloses separating straight chain from non-straight chain C.sub.6 -C.sub.10 hydrocarbons and separately converting to aromatics; and U.S. Pat. No. 4,358,364 which discloses catalytically reforming a C.sub.6 fraction and producing additional benzene by hydrogasifying a C.sub.5- fraction, a fraction with a boiling point above 300.degree. F. and the gas stream produced from catalytic reforming.
U.S. Pat. No. 3,753,891 discloses fractionating a straight run naphtha into a light naphtha fraction containing the C.sub.6 and a substantial portion of the C.sub.7 hydrocarbons and a heavy naphtha fraction boiling from about 200.degree. to 400.degree. F.; then reforming the light fraction to convert naphthenes to aromatics over a Pt-alumina catalyst or a bimetallic reforming catalyst; separately reforming the heavy faction, then upgrading the reformer effluent of the low boiling fraction over a ZSM-5 type zeolite catalyst to crack the paraffins and recovering an effluent with improved octane rating.
U.S. Pat. No. 4,645,586 discloses parallel reforming of a hydrocarbons feed. In one stream, the hydrocarbons are reformed with an acidic catalyst. In the second stream, the hydrocarbons are reformed with a non-acidic catalyst.
U.S. Re. Pat. No. 33,323 discloses solvent extraction of a light fraction of a reformate. A hydrocarbon feed is separated into a lighter fraction and a heavier fraction. The lighter fraction is reformed in the presence of a non-acidic catalyst. The heavier fraction is reformed in the presence of an acidic catalyst. The reformate from the non-acidic catalyst is introduced into an extraction where an aromatic extract stream and a non-aromatic raffinate stream are recovered. The raffinate stream can be recycled to the feed. The aromatic extract stream contains 30% benzene, 18% toluene, and 51.8% C.sub.8 aromatics.
Solvent extraction is well known as a means of removing aromatics from a reformate stream.
U.S. Pat. No. 2,956,005 discloses solvent extraction of a light fraction of a reformate. That light fraction may have an end point in the range of 220.degree. to 300.degree. F., or it can be a C.sub.6+, C.sub.7, or C.sub.7+ fraction. The resulting raffinate is recycled to the reforming zone. The reforming catalyst used is a non-zeolitic catalyst.
U. S. Pat. No. 3,121,676 discloses solvent extraction of a C.sub.7- fraction of a reformate. A debenzenizer is used after solvent extraction to remove toluene from the final product. The reforming catalyst used is a non-zeolitic catalyst.
U.S. Pat. No. 3,280,022 also discloses solvent extraction of a C.sub.7- fraction of a reformate. A debenzenizer is used after solvent extraction to remove toluene from the final product. The reforming catalyst used is a non-zeolitic catalyst.
U.S. Pat. No. 4,358,364 discloses solvent extraction of a light fraction of a reformate. That light fraction is a benzene-rich fraction. The heavier fraction is a toluene-xylene-rich fraction. The reforming catalyst used is a non-zeolitic catalyst.
U.S. Pat. No. 4,648,961 discloses reforming a hydrocarbon feed with a non-acidic zeolitic catalyst, separating an aromatics product from a gaseous stream, separating by solvent extraction normal paraffins and a substantial portion of the single branched isoparaffins from the aromatics, and recycling the normal and single branched isoparaffins to the reforming vessel.
Extractive distillation in general is well known in the art. For example, U.S. Pat. No. 3,434,936 to Luther et al discloses the separation of a mixture of hydrocarbons to aromatic and non-aromatic components by extractive distillation using N-substituted morpholines. U.S. Pat. No. 3,795,588 to Preusser et al discloses a process for separating and recovering diolefins from C.sub.4 -C.sub.5 hydrocarbon mixtures containing paraffins, monoolefins, and diolefins by using an extractive distillation technique.